The polo-like kinase (PLK) family of serine/threonine kinases comprises at least four known members: PLK1, PLK2 (also known as Snk), PLK3 (also known as Fnk or Prk) and PLK4 (also known as Sak). Agents which inhibit PLK4 have the potential to treat cancer. A number of potent PLK4 inhibitors are disclosed in U.S. Pat. Nos. 8,263,596, 8,481,525, and 8,481,533 (the entire teachings of which are incorporated herein by reference). The structure of one inhibitor disclosed in these patents is shown below as compound (I):

There is a need for salt forms of this compound that are crystalline and otherwise have physical properties that are amenable to large scale manufacture. There is also a need for pharmaceutical formulations in which this drug candidate is stable and is effectively delivered to the patient.